Millennia ago, as the great glaciers of the Ice Age receded, methane that had been frozen under the seafloor melted and burst forth, leaving massive craters that remain on the ocean bottom to this day.

Today, new research suggests humankind should be concerned that a similar release of methane, a potent greenhouse gas that can harm marine ecosystems, might happen again as global warming melts the ice in the Arctic, Greenland, and elsewhere.

“We provide a model for abrupt and explosive methane releases,” said Karin Andreassen, a professor at CAGE Centre for Arctic Gas Hydrate, Environment, and Climate and the lead author of a study on Arctic methane published in the journal Science.

Examining craters formed by methane eruptions 12,000 years ago, Andreassen and her colleagues determined that enormous amounts of methane in hydrate form — an ice-like mix of water and gas — must have dissolved as the 6,500-feet-thick ice sheets retreated from the Barents Sea north of what is now Scandinavia and western Russia.

They found more than 100 craters on the seafloor that ranged from around 1,000 to almost 3,300 feet wide — larger than any discovered before — as well as thousands of smaller pockmarks, according to their research.

“The principle is the same as in a pressure cooker,” she said. “If you do not control the release of the pressure, it will continue to build up until there is a disaster in your kitchen. These mounds were over-pressured for thousands of years, and then the lid came off.”

The study area is in Bjørnøyrenna close to the Arctic archipelago Svalbard. |
K. Andreassen/CAGE

It’s not clear how much of that methane reached the atmosphere, where it traps 84 times more heat in the atmosphere than carbon dioxide over a 20-year period, according to the Environmental Defense Fund.

Today, methane seepage is common throughout the oceans. Researchers believe bacteria consumes most of that methane, however, converting it into other chemicals like carbon dioxide that causes coral bleaching and other problems but also prevents it from reaching the surface to contribute to global warming.

Andreassen is now calculating the pace of the eruptions the caused the craters — she suspects they took place over the course of months or a year — and the amount of methane they disgorged.

Those numbers will be key to determining whether to be alarmed about this new research, said University of Washington Oceanographer H. Paul Johnson, who has studied methane in waters off the Pacific Northwest. Unfortunately, he added, humans might soon witness the lid coming off a methane pocket. The conditions that melted glaciers in the past are occurring now.

“The only way you can keep this hydrate that’s in the ground is to keep from warming the oceans,” said Johnson. “The only way you can keep them from warming is to reduce the greenhouses gases in the atmosphere.”